1. Field of the Invention
The present invention relates generally to packages for MEMS (Micro Electro Mechanical System) and manufacturing the same. More particularly, this invention relates to packages for MEMS devices by sealing the MEMS portion of device hermetically to keep the inside of package dry to avoid the stiction of moving parts. The combination of printed circuit board (PCB), transparent substrate, and MEMS device enables the next generation of MEMS display devices. Flip-Chip packaging of MEMS device using Printed Circuit Board (PCB) and transparent substrate is described here.
2. Description of the Relevant Art
Micro-mirror devices have drawn considerable attention in their use as Spatial Light Modulators (SLM). A spatial light modulator requires an array of a relatively large number of such micro-mirror devices. In general, the number of devices required ranges from 60,000 to several million for each SLM. Despite significant advances that have been made in recent years, there is still a need for improvement in the performance and manufacturing yields of electromechanical micro-mirror devices.
One critical engineering challenge encountered by those of ordinary skill in the art is in the connection of the MEMS-CMOS system (mirror system) to a printed circuit board interface that is easily mounted to enable the transfer of power and data as the interface between the power source and image signal input device. Specifically, a typical device employs a packaging configuration generally known as wire bonding wherein individual wires are connected between the mirror system and the interface for receiving power and signals transmitted through these bonding wires.
One limiting factor for implementing the wire-bonding packaging configuration arises from the physical space required for connecting and extending a large number of bonding wires between a miniaturized mirror device and the interface connection terminals. The space limitation is even more critical now because there is significant demand for smaller micro-mirrors, so called Pico-projectors. For such small mirror device, the physical space required to transfer data is limited by the small size of the mirror system. There is also significant demand to increase the amount of data transferred to the mirror system because the amount of data transferred often directly impacts the ability to create greater depth of brightness (pixel bit-rate) as well as the number of pixels on the image to generate images of high resolution. In order to achieve the image display of high image quality either by implementing smaller micro-mirror devices to achieve miniaturized device size, or implementing micro-mirror devices with higher resolution and increasing bit-rate, the space limitation becomes a bottle neck. For the purpose of satisfying current trends of image display, there is a need that requires alternative packaging configuration for electrically connecting the mirror system to an interface.
Another limiting factor arises from the cost of production required for wire bonding. Each pad on the MEMS-CMOS must be physically connected to a counterpart on the interface. The wire bonding process is now mechanized, but the process still limits production yield and the complicated structure causes the production cost to increase. For these reasons, wire bonding may still be a viable packaging configuration of producing micro-mirror devices if the number of wires is relatively low. However, as the need for data transfer increases, wire bonding is no longer a cost effective or technically feasible configuration of producing micro-mirror devices.
One solution to form the electrical connections between a mirror system to an interface is a packaging methodology called flip-chip packaging. In this methodology, bumps are placed on each mirror system pad that conventionally required a wire bonding connection and pads on the interface are approximated to the solder balls. The electrical connection is formed by first heating the solder balls followed by removing the heat for cooling down and solidifying the connections.
However, there are also limiting factors when the flip-chip configuration is implemented for forming the connections of the mirror devices. When the heat is applied to the bumps to make a permanent electrical connection between the mirror system and interface, both systems undergo a thermal expansion with the rise of the temperature. A problem is caused by the mismatch of thermal expansions between the mirror system that is made primarily of silicon and the interface that is most commonly formed on a printed circuit board, often made with plastic or fiber reinforced plastic. Specifically, the rate of heat expansion of silicon and plastic differ, an electrical coupling made at high temperatures will sense a shearing tension as the materials cool, ultimately breaking the coupling.
One method of circumventing this problem is to employ a Chip-on-Glass (print patterning of electrically conducting material on a glass substrate) and use flip-chip packaging to connect the mirror system to the glass substrate. Since glass and silicon have approximately the same heat expansion rates, no significant shearing tension is felt and the electrical couplings remain intact. However, printing circuits on glass is an expensive process and although this process may produce one or few well performed device as development samples, such processes have not matured into mass production manufacturing processes in order to produce commercially viable product.
For these reasons, there are still needs in the art of manufacturing image display devices to provide new and improved packaging configuration and manufacturing processes for MEMS device. The new and improved configuration and manufacturing processes must be able to produce micromirror devices that can transfer large amounts of data with limited physical space and can be manufactured with simplified process and lower costs such that the above discussed limitations and difficulties can be resolved.